The present invention relates to a process for converting zinc chloride to zinc oxide during the hydrometallurgical processing of electric arc flue dust (EAFD) to recover the valuable base metals from the flue dust while recycling the iron. More particularly, the present invention relates to a process using combination of calcium hydroxide at an elevated temperature and pressure under an inert atmosphere to produce essentially chlorine-free zinc oxide at substantially 100% yield.
Electric arc processing of recycled steel is the primary method used today in the United States to produce new steel. Electric arc processing replaces the conventional Bessemer furnace that arose at the glory days of the steel age at the turn of the Twentieth century and continued in use throughout this century. The electric arc process is more efficient in terms of energy and labor than the traditional mill processes. It produces, however, an electric arc flue dust (EAFD) waste product that is depleted in iron while remaining relatively rich in other commodity base metals, like zinc. This flue dust waste plagues the industry because it is unstable and is difficult to economically process to create a stable waste or to recover the base metals. Furthermore, EAFD is classified as a hazardous waste under U.S. EPA standards, so few facilities will store or process EAFD. The primary process used to treat EAFD is the Horsehead Industries pyrometallurgical process that is energy intensive in recovering the base metals. This pyrometallurgical process, however, itself produces flue dust which is even more difficult to stabilize because it is depleted of both iron and other base metals. Nevertheless, electric arc furnaces pay the pyrometallurgical processor today to process the EAFD so that the electric arc furnaces can continue to recycle iron and steel into new products. Obtaining such treatment for the EAFD requires transport of the dust from the electric arc furnace to the pyrometallurgical processor, which introduces yet another cost. The industry would benefit greatly from a process that would allow the electric arc furnace operator or another associated processor to treat and stabilize the EAFD at the electric arc furnace to recycle iron to the furnace while both recovering base metals for resale and eliminating unstable metal dusts or tailings.
The problem with untreated EAFD in the United States is substantial. Many tons of EAFD are presently stored at the existing furnace operations with EAFD being produced at a rate of about 500,000-750,000 tons/yr to more than one million tons per year by some estimates. The pyrometallurgical processor can only process about 250,00-300,000 tons/yr so the problem of EAFD continues to grow, adding about 250,000-500,000 ton/yr or more to the increasing stockpile. This remaining flue dust currently is either sent to Mexico (a processing opportunity today that may be curtailed by NAFTA environmental requirements) or is buried in hazardous waste landfills. Both of these alternatives are very expensive and do not resolve the environmental concerns. They both waste the metal value of the EAFD. Environmental concerns may soon curtail the use of the electric arc furnace in the U.S. unless an effective process is discovered for treating the EAFD. Slowdown or stoppage of the electric arc furnaces will create a problem with disposal of the iron and steel that presently is being recycled as well as a dramatic price increase for steel.
The existing pyrometallurgical process for treating EAFD is not the long-term solution because of the problems associated with it. This pyrometallurgical process is energy intensive and itself creates a flue dust waste that is even more troublesome, albeit in smaller quantities, than the EAFD from which it starts. Economics and environmental concerns do not permit expansion of the pyrometallurgical process, and, in fact, they too threaten its existence. Therefore, the gap its growing with the profitability and vitality of the U.S. steel industry hanging in the balance.
Recovery of metals (both the base metals, like copper, lead, and zinc, and the precious metals, like gold and silver) from ores and concentrates, fly ash (including sewer sludge ash), contaminated soils, flue dust from base metal smelters and electric arc furnaces, and the like has been a significant commercial interest for many years. The precious metals are elements of wealth. The base metals are important in modern manufactured products. Existing processes have their merits and their drawbacks. Until the recent surge of environmentalism, recovery of precious and base metals drove the selection of the processes used. The important factor was simply obtaining these metals and the environmental costs were largely ignored. Society tolerated the resulting air, water, and soil pollution. Today, however, there is interest in developing a process that balances the needs of industry and the environment. There is need for a process that recovers the metals with high yield (generally at least about 85 wt. %) without producing hazardous wastes. We need a process that eliminates hazardous fluid waste streams, thereby greatly reducing water and air pollution that otherwise accompanies xe2x80x9csmelting.xe2x80x9d The processes of the past have been tried and rejected, forgotten or discarded because they failed to satisfy all the important criteria. Emblematic of the change that has occurred in the metal processing industry is the shift to the electric arc furnace itself where metal hulks are recycled into new products at lower cost and with less pollution than can be achieved with recovering iron from ores.
The hydrometallurgical recovery process of U.S. patent application Ser. No. 08/376,778, which we incorporate by reference, offers a process that is efficient, economical, and environmentally-sensitive. It produces an iron-rich feedstock for an electric arc furnace, recovering the valuable base metals in the EAFD, and producing calcium sulfate (gypsum) suitable for building materials without creating significant air, water, or solid wastes while eliminating the problems of the prior art processes and achieving the goals of today.
The hydrometallurgical process of U.S. patent application Ser. No. 08/376,778 for treating EAFD uses a calcium chloride/hydrochloric acid leach to place the soluble base metals (i.e., copper, lead, cadmium, and zinc) into chloride solution generally without permitting a significant amount of the iron into solution. The chemistry of the process takes advantage of the relatively lower solubility of iron oxides (Fe2O3, Fe3O4) to that of the base metals in moderately acidic chloride solutions. The process also uses an oxidizing atmosphere to oxidize any soluble ferrous iron to ferric iron, which is essentially insoluble in aqueous solutions at a pH greater than 2.0. Thus, iron that does enter solution in the leach reactor oxidizes to the ferric form and precipitates out to be recovered with the iron-rich waste cake. The metal-rich solution that results from the leach is then treated using zinc dust to precipitate the copper, lead, and cadmium, followed by calcium hydroxide (hydrate of lime) to recover the zinc before the hydrochloric acid/calcium chloride leach mill solution is replenished or regenerated with sulfuric acid and recycled for reuse.
A slurry of the flue dust and the leach mill solution having a solids content (i.e., pulp density) of about 15-30 wt. % is reacted at a pH of about 2.6, at a moderately elevated temperature of about 90-120 deg C., at an elevated pressure of about 90 psi in an oxygen-rich atmosphere. While we can use higher pressures, they do not offer a significant advantage in improving the speed of the reaction or the yield of metals, so we avoid incurring the costs associated with operating at higher pressures. So, too, with the temperature. We prefer to use oxygen in our process to ensure that any soluble iron is converted to the ferric state and so that all other soluble metals are in their highest oxidation states.
We minimize the amount of soluble iron (having as little as 10 mg/l in the metal-rich solution) so that essentially all the iron will be recycled to the electric arc furnace with the waste cake and so that subsequent recovery and purification steps for the soluble base metals will not be complicated by the presence of iron.
In the process, then, we preferably mix dry EAFD powder with the leach mill solution in a ball mill to produce a slurry having the desired pH and solids content. We then oxidize the base metals, if necessary, in the slurry, other than the iron, to make them soluble. This leach occurs in the oxygen-rich atmosphere at a temperature of about 90-120 deg C. (preferably 120 deg C.) and a pressure of about 50-90 psi (preferably 90 psi) and, if the pH is controlled at about pH 2.6, leaves an insoluble hematite complex that can be separated and that can be recycled to the electric arc furnace for recovery of the iron. The zinc-containing complexes readily entering solution by this process are ZnO and ZnS, if present. xe2x80x9cZinc ferritexe2x80x9d species do not readily dissolve under the conditions of the present invention and, consequently, this zinc remains in the iron-rich waste cake. The metal-rich solution is treated to precipitate the metals of interest separately or together to permit their recovery. We prefer a zinc cementation precipitation of the copper, lead, and cadmium, concluding with the precipitation of zinc oxide by the addition of calcium oxide, calcium hydroxide, or a mixture thereof (preferably calcium hydroxide) leaving a calcium chloride solution having a pH in the range of about pH 6-10, and, generally around pH 6 unless an excess of calcium hydroxide is used during the zinc oxide precipitation. We treat this solution with sulfuric acid to create a solid gypsum byproduct and the leach mill solution which we recycle for further treatment of EAFD. Since we want the leach mill solution to have a pH of about 2.0-3.0, we want to minimize the pH of the calcium chloride solution while recovering substantially all of the zinc. Therefore, we prefer to add the minimum amount of calcium hydroxide necessary to promote the zinc oxide precipitation.
The economic success of the hydrometallurgical process of U.S. patent application Ser. No. 08/376,778 hinges in large measure upon the recovery of the base metals, especially the zinc, in a salable form. Therefore, according to the process of the present invention, we produce substantially chlorine-free zinc oxide in substantially 100% yield by reacting the zinc chloride solution with a calcium compound in a reaction that we call the xe2x80x9czinc metastasisxe2x80x9d reaction.
The xe2x80x9cmetastasisxe2x80x9d process of the present invention for the recovery of zinc in the treatment of EAFD involves the reaction of calcium hydroxide or calcium oxide with a zinc-rich solution at a temperature of about 150 deg C. for about 1 hour at a pH of about 6-10, and, preferably at a pH as low as possible to conserve acid in later EAFD processing steps. To minimize the amount of lime required to adjust the pH, we try to minimize the volume of the solution. Still, in processing large volumes of EAFD, the volumes are substantial. We add about a stoichiometric amount of hydrated lime based upon an analysis of the zinc concentration in the solution, producing a solution with a pH between about 6-10. That is, we add the theoretically correct amount of lime to recover the zinc, and we have found that our process is high yield. The metastasis process produces a zinc oxide precipitate (ZnO often with Zn(OH)2) and a calcium chloride solution suitable for recycling to the EAFD leach when its pH is adjusted. Because the pH of this solution is about 10.5, to reuse the calcium chloride solution, we need to add acid, especially sulfuric acid, which is advantageous for several reasons. The ZnO/Zn(OH)2 product that we prepare is suitable for further purification or processing by conventional methods.